This isn't the old "plenum rated" vs. "in-wall rated" thing again, is it ? I'm still downloading WU updates (second day of downloads now) so everything is terribly, awfully slow or I would search myself.
My recollection from the last discussion was that you needed special insulation on the wire if it was going to be used either inside air ducts, or inside a section of the wall where air was flowing as part of an HVAC system. Otherwise, in wall did not need anything special.
I also have a vague recollection of being proven wrong by someone who actually knew something about construction, so take this with a grain of salt
